1. Field of the Invention
The present invention generally relates to long-period fiber gratings and to methods for reducing polarization dependent loss of a long-period fiber grating and for shifting the peak wavelength of a long-period fiber grating. More specifically, the present invention relates to long-period fiber gratings designed for use as a gain-flattening filter with an optical amplifier.
2. Technical Background
Fiber optic networks transmit a plurality of optical signals of different wavelengths through a single fiber employing wavelength division multiplexing. Losses result in the decreasing of intensity of such optical signals as they propagate over significant distances. Thus, when transmitting optical signals through an optical fiber over long distances, the fiber is typically divided into spans, with in-line optical amplifiers positioned between the spans to periodically amplify the intensity of the transmitted optical signals. A typical span is, for example, 80 km in length. The in-line optical amplifiers commonly used for such purposes generally exhibit optical signal gains that are different for the different wavelengths of the transmitted optical signals. Thus, each time the transmitted optical signals are amplified by one of the in-line optical amplifiers, certain optical signals are amplified to a greater degree than other optical signals at different wavelengths. Accordingly, when many in-line amplifiers are used over a long distance, significant discrepancies between the intensities of the transmitted optical signals may exist.
To overcome this problem, gain-flattening filters have been developed that may be used with an amplifier to flatten the gain across the spectrum of optical signals that are transmitted through the amplifier. One such gain-flattening filter is a long-period fiber grating (LPG).
LPGs function to attenuate intensity levels of the optical signals transmitted through the LPG dependent upon the wavelength of the optical signals. Thus, for optical signals of a first wavelength, the LPG may attenuate the intensity of those optical signals to a greater or lesser degree than it may attenuate optical signals at other wavelengths. This selective attenuation is accomplished by coupling some of the light of the optical signals that is transmitted through the core of the fiber into the cladding of the fiber. Once this light is in the cladding of the fiber, it then dissipates through the fiber coating so that this light is permanently lost. The peak wavelength xcex for which coupling occurs from the core mode to the cladding mode in an LPG is:
xcex=(neffcorexe2x88x92neffclad)xcex9
where neffcore is the effective index of the core mode, neffclad is the effective index of the cladding mode, and xcex9 is the grating period.
FIG. 1 shows the spectral characteristics for a typical LPG, which is represented on a dB scale. As shown in FIG. 1, the spectral characteristics have a generally Gaussian shape, with some ripples on the sides. When used in a gain-flattening filter, a plurality of such LPGs is provided in series each having differing peak wavelengths than the others. In such a structure, the spectral characteristics accumulate resulting in a relatively complicated loss spectrum that complements the spectral gain characteristics of the in-line amplifier with which the gain-flattening filter is utilized.
LPGs are exceptionally sensitive to variations in fiber properties including core radius and core and clad refractive index. Typically, small changes in these parameters manifest themselves as wavelength shifts of the grating loss peak. An additional complication is introduced by small asymmetries in the fiber geometry that lead to birefringence. In gain flattening filter applications, the peak wavelength of an LPG must be controlled within as little as 0.1 nm depending on the particular filter. This corresponds to an neff difference on the order of xcx9c5xc3x9710xe2x88x927. It is extremely difficult to control the fiber parameters (refractive index profile and core dimensions) well enough to reproduce grating peak wavelengths to within 0.1 nm. There are additional inconsistencies in fiber photosensitivity that manifest themselves as a different wavelength shifts upon grating annealing. Typical variations in peak wavelengths are on the order of 1 nm or more. To obtain a reasonable yield of devices, it is therefore desirable to develop a method of tuning the grating wavelength after the writing and annealing processes.
Due to the extreme sensitivity of LPGs to fiber parameters, the effective index neffclad of the core mode changes significantly with polarization even in a relatively low birefringence fiber. This results in a peak wavelength shift of the LPG for different polarizations. Thus, optical signals polarized in one direction will be affected differently than optical signals polarized in a different direction when propagating through the LPG. This difference produces a polarization dependent loss (PDL), which is illustrated in FIG. 2 for a case in which the peak wavelength changes by 0.2 nm as a function of polarization in a typical grating. It should be noted that PDL is defined as a positive number and the graph in FIG. 2 plots the absolute value of the difference between a pair of shifted spectra.
Research has revealed that PDL in LPGs can vary over a tremendous range of less than 0.1 dB to more than about 1 dB, depending on the type of fiber and the type of filter. From FIG. 2, it is apparent that the PDL is proportional to dL/dxcex, where L is the grating loss. This implies that filters that have sharp features are more susceptible to PDL. Therefore, there is also a need for a method for reducing PDL after the grating has been written in the fiber.
Accordingly, it is an aspect of the present invention to provide a method of reducing PDL in a LPG. It is also an aspect of the present invention to provide a method for adjusting the peak wavelength of the light propagating through a LPG that is coupled into the cladding of the fiber. Another aspect of the present invention is to provide an LPG that has a characteristic peak wavelength that may be adjusted after its manufacture and that has a reduced PDL.
To achieve these and other aspects and advantages, a method according to a first embodiment of the present invention that reduces PDL in an LPG comprises the step of twisting the LPG.
According to another embodiment of the present invention, a method is provided for adjusting the peak wavelength of light propagating through an LPG that is coupled into the cladding of the fiber, which comprises the step of twisting the LPG until the desired adjustment to the peak wavelength is obtained.
According to another embodiment of the present invention, a method for manufacturing an LPG is disclosed that comprises the steps of providing a fiber having a core surrounded by a cladding, the fiber having two ends and extending therebetween along a longitudinal axis, writing a long-period grating pattern onto a portion of the fiber, annealing the fiber, and twisting the fiber throughout at least the portion of its length that includes the long-period grating pattern.
According to another embodiment of the present invention, a long-period fiber grating comprises a fiber having a core surrounded by a cladding, the fiber having two ends extending therebetween along a longitudinal axis, and a plurality of refractive index variations periodically spaced along the longitudinal axis of a portion of the core of the fiber, wherein the fiber is twisted throughout at least a portion of its length that includes the plurality of refractive index variations.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.